GEOCHEMISTRY OF AMPHIBOLITES AND QUARTZOFELDSPATHIC GNEISSES IN THE PAN-AFRICAN ZAMBEZI BELT, NORTHWEST ZIMBABWE - EVIDENCE FOR BIMODAL MAGMATISM IN A CONTINENTAL RIFT SETTING

The Zambezi belt separates the Congo and Kalahari cratons in southern Africa and is a key part of the regional Pan-African orogenic framewor k related to amalgamation of Gondwana in the Neoproterozoic-early Pala eozoic. Several thick, probably correlative, supracrustal sequences ar e preserved in the belt in Zimbabwe and Zambia. The Makuti Group, a ma jor assemblage of supracrustal rocks within the belt in northwestern Z imbabwe, consists dominantly of amphibolite-facies quartzofeldspathic gneisses of supracrustal origin interlayered with horizons of marble, calc-silicate rock, quartzite, and pelitic schist. Numerous thick, con cordant amphibolites derived from mafic sills and/or lava flows are in tercalated within the supracrustal sequence. Major- and immobile trace -element geochemistry indicates dominantly tholeiitic affinities for t he amphibolites, with some samples showing transitional to alkaline af finities. High-field-strength trace-element contents and LREE-enriched patterns are consistent with a within-plate setting for the mafic roc ks. Major- and trace-element data show the quartzofeldspathic gneisses to be dominantly of igneous origin. Their protoliths are inferred to be mainly peralkaline rhyolites and trachytes. High Zr contents (up to 1500 ppm) are a diagnostic signature for these rocks. The bimodal nat ure of the magmatism and the abundance of peralkaline felsic rocks poi nt to a continental rift zone as the setting for the Mahuti Group. Oth er examples of pre-orogenic, mafic or bimodal magmatic rocks are found in the Zambezi belt elsewhere along strike in Zambia and Zimbabwe. Al l these rocks are inferred to represent widespread, rift-related magma tism associated with initiation of the depositional basin within which the Neoproterozoic sequences of the Zambezi belt accumulated.